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Yogurt Connoisseur Wiki
What is Yogurt? Yogurt is a milk product that is produced through bacterial fermentation. The name, yogurt, itself is a broad classification of several milk products that share a similar preliminary step of bacterial fermentation. From here, several techniques are used to modify the fermented milk product for different culinary applications. Furthermore, certain types of yogurt is claimed to have several positive effects on overall health through a variety of research methodologies . __TOC__ How is it made? In most cases, cow's milk is used. However, this can be substituted for any mammalian milk sources. Since fat in raw milk normally settles on the top, the milk is first homogenized and then pasteurized. The next step involves innoculating milk with a bacterial culture and incubated at around 42 degrees Celsius until the product is firm. This change in viscosity occurs due to lactate production during anaerobic fermentation. Yogurt texture is only one characteristic that plays into yogurts entire sensory evaluation of taste, acidity, smell and appearance. (If you're interested in producing yogurt at home, simply purchase milk at your grocery store and you're ready to start. Chances are that it is already homogenized and pasteurized.) Blah Blah Blah Testing Homogenization This is the name of any process that is utilized to mix two non-miscible solutions until evenly mixed. This is usually done by converting one solution into extremely small particles that can be dispersed within the other solution. Several methods exist to homogenize milk, but one of the most common ones is to force fresh milk through small holes at high pressure, thus breaking fat globules into smaller particles and dispersing them within the solution. Pasteurization Pastuerization is the process of heating foods to a specific temperature for a certain amount of time before cooling. Several pasteurizing techniques exist such as the high temperature, short time (HTST) method. HTST involves heating milk to 72 degrees Celsius for 15 seconds before cooling to 4 degrees Celsius. HTST has been shown to kill 99.999% of bacteria found within milk Stabel, J. R.; Lambertz, A. (2004), "Efficacy of Pasteurization Conditions for the Inactivation of Mycobacterium avium subsp. paratuberculosis in Milk", Journal of Food Protection 67 (12): 2719 . However, some claim that this removes some of the probiotic and natural enzymes found within milk leading to a large controversy over the usefulness of pasteurization. It is important to note that this pasteurization process significantly increases the shelf life of milk and is vital for the production of yogurt through preventing contaminating bacterial growth within the milk. How do we assess the quality of yogurt? The sensory evaluation of the final product should be prioritized over any other type of analysis. From a technical standpoint, the researchers could only do a sensory evaluation, as there are no standards for testing the physical properties of the yogurt. The researchers were attempting to study the effect of a LT-ROF production on the physical, sensory, properties, such as taste and consistency of the yogurt.1. Horiuchi, H., Inoue, N., Liu, E., Fukui, M., Sasaki, Y., and Sasaki, T. (2009) A method for manufacturing superior set yogurt under reduced oxygen conditions.'' J Diary Sci. 92,' 4112--4121 Ultimately, the researchers are testing a new production method for yogurt, and if the yogurt is not pleasing to one’s palate, the product will not sell, and the experimental production method is a failure regardless of what any data may say. The fact that the researchers used a large pool of consumers who regularly consume yogurt is a major positive, as the sensory evaluation from these individuals will be well-informed due to the individuals’ love and experience with yogurt. Thus, even though a sensory evaluation is subjective, it is the most important form of evaluation in any study regarding the production of foods. A bad tasting product will simply not sell, dooming it to market failure and thus the failure of the study's ultimate goal. Importance of Bacterial Metabolism to Yogurt Culture Both ''S. thermophilus and L. bulgaricus are facultative anaerobes, meaning that they have the ability to oxidize glucose using molecular oxygen if it is availableZourari, A., Accolas, J. P., and Desmazeaud, M. J. (1992) Metabolism and biochemical characteristics of yogurt bacteria. A review. Le Lait 72, 1–34. However, the conditions under which yogurt is made are generally anaerobic, requiring that both species shift their metabolism towards fermentation. In fact , both species grow best in anaerobic conditions, in part due to their inability to handle reactive oxygen species generated by aerobic metabolism Higuchi, M., Yamamoto, Y., and Kamio, Y. (2000) Molecular biology of oxygen tolerance in lactic acid bacteria: Functions of NADH oxidases and Dpr in oxidative stress. Journal of Bioscience and Bioengineering 90, 484–493 Condon, S. (1987) Responses of lactic acid bacteria to oxygen. FEMS Microbiology Letters 46, 269–280. The role of starter cultures in yogurt production is primarily to acidify milk by converting lactose into lactateLee, W. J., and Lucey, J. A. (2010) Formation and physical properties of yogurt. Asian-Aust. J. Anim. Sci 23, 1127–1136'. The disaccharide lactose can be degraded to glucose and galactose, with the latter sugar entering glycolysis via the Leloir pathway. Carbon shuttled into glycolysis ultimately finds itself in the form of pyruvate. The organism uses the free energy difference between glucose and pyruvate to synthesize ATP and reduce NAD+ to NADH. In the absence of molecular oxygen as a final electron acceptor, the electron transport chain is unable to reoxidize NADH to NAD+, upon which glycolyis is dependent. To ensure the regeneration of NAD+ and the continual production of ATP from glycolysis, pyruvate is reduced to lactate using NADH as an electron donor. As lactate builds, it diffuses outside of the organism. To maintain electroneutrality, protons diffuse out in equal proprtions, thus acidifying the milk. Depending on the nature of the heat treatment, the milk undergoes gelation around pH 5.3, creating a firmer texture . Synergism between Streptococcus thermophilus '' and ''Lactobacillus delbrueckii ''subsp. bulgaricus ''Streptococcus thermophilus (S. thermophilus) and Lactobacillus delbrueckii subsp bulgaricus (L. delbrueckii subsp bulgaricus) are able to acidify milk, providing its sharp, acidic taste and flavour.Zourari, A., Accolas, J. P., and Desmazeaud, M. J. (1992) Metabolism and biochemical characteristics of yogurt bacteria. A review. Le Lait 72, 1–34 Both bacteria are facultative anaerobes, meaning that they are able to make ATP by both aerobic respiration and fermentation. Streptococcus thermophiles and Lactobacillus delbrueckii subsp bulgaricus synergistically work together to stimulate their growth and acid production, through a phenomenon known as proto-cooperation. Their combined efforts often report in high acidication rates, a lower final pH, and an increased''S. thermophilus'' population, promoted aromatic compound production and increased stability of final product compared to monocultures. These two strains are able to work together by exchanging growth stimulants such as folic acid and carbon dioxide to supplement for the absence of various nutrients. In 2009, Herve-Jimenez et al.''conducted a study to investigate the biochemical interactions between the two bacteria through post-genomic analyses on a coculture of the two strains.Herve-Jimenez, L., Guillouard, I., Guedon, E., Boudebbouze, S., Hols, P., Monnet, V., Maguin, E., and Rul, F. (2009) Postgenomic analysis of streptococcus thermophilus cocultivated in milk with Lactobacillus delbrueckii subsp. bulgaricus: involvement of nitrogen, purine, and iron metabolism. Appl. Environ. Microbiol. 75, 2062–2073 Total RNA was extracted from the cultures at the 2.5 and 5.5 hour marks, and genome-wide analyses were performed on the extracted RNA using a commercial DNA microarray kit able to detect the genes present in the two strains. The pH and species-specific bacterial counts were also quantified every hour. It was found that when the two bacteria were cultvated together, the acidification of milk and bacteria counts were enhanced (see Figure 2). It was also found that in coculture, there was an upregulation of peptides, amino acid transporters and amino acid biosynthetic pathways compared to coculture. Specifically, sulfur amino acid and galactose metabolism, the purine biosynthesis pathway, iron metabolism, and arginine (Arg) and Branched Chain Amino Acids (BCAA) biosynthetic pathways were stimulated. To illustrate the symbiotic nature of the two strains, consider the upregulated Arg and BCAA biosynthetic pathways. Arg and BCAA are required for optimal growth in milk, particularly for protein synthesis. This biosynthetic pathway is not present in ''L. delbrueckii' subsp ''bulgaricus', however, is present in ''S. thermophilus. L. delbrueckii ''subsp ''bulgaricus ''evidently benefits from being in coculture with S. thermophilus''. Surprisingly, S. thermophilus'' also benefits from the presence of L. delbrueckii subsp ''bulgaricus ''because depletion of its amino acid stores forces the strian to upregulate amino acid biosynthesis in S. thermophilus, allowing for the enhanced growth of this strain. Link Between Type II Diabetes and Dairy Consumption Recently, there has been studies that have suggested a relationship between diary consumption and incidence of type II diabetes. In 2005, Hyon et al. utilized a large prospective cohort study to demonstrate a link between the incidence of type II diabetes (T2D) and consumption of dairy products among men. Particularly, the authors found a significant reduction in risk of developing T2D within men who are in the top quintile of dairy consumption (RR=0.77).Choi HK, Willett WC, Stampfer MJ, Rimm E, and Hu FB (2005) Dairy consumption and risk of type 2 diabetes mellitus in men: A prospective study. Arch Intern Med 165, 997–1003 While its findings are of significance, one must be careful when drawing clinical conclusions from such data. The type of study is of significant concern, especially if used to influence clinical practice. Observational studies, while useful, do have significant limitations, such as the significant chance in selection bias due to the lack of randomization. It would be imprudent to derive clinical conclusions from the results of this study alone. However, given the impossibility of randomizing an exposure (dairy consumption), prospective observational data may be the best form of evidence available. Another area of concern is the lack of sample calculations to demonstrate that the study has sufficient power to make the conclusions that it has made. Instead, the authors merely stated that their study had a large sample size with no reference to statistical power or error. One other source of major error is that diagnosis of diabetes was achieved by self-reporting with a followup chart review. Under-diagnosis is of significant concern as screening for serum glucose is not always performed. To decrease bias, the data collectors and analysts should have been blinded to the patient’s exposure status. There is no evidence of this occurring. Finally, this study lacks external validity as only men between the ages of 40 and 75 with no prior history of the diseases were used as subjects, and thus any conclusions can only be generalized to this group. Whether if these findings apply to women or prevalent cases remains to be seen. One area in which the authors have succeeded is in the demonstration of a reasonable attempt at adjusting for confounding variables such as BMI, family history, and physical activity, which increases the internal validity of this study. Despite the study’s conclusion of an appreciable difference in relative risk between groups, the reader must not infer causality and use caution in allowing this study to influence clinical practice. The difference in RR, while detectable, is not sufficient to imply causality. Given the impracticality of an RCT, the demonstration of a dose-response relationship, or the proposal of a pathologically plausible mechanism, could be used to imply causality. These could be areas of further research to bolster the results of this prospective cohort study. Health Benefits of Probiotic Yogurt For health-conscious consumers, probiotic yogurt has been shown to have a stimulatory effect on cellular immunity, reduce cholesterol levels, and promotes intestinal tract health.Lourens-Hattingh, A., and Viljoen, B. C. (2001) Yogurt as probiotic carrier food. International Dairy Journal 11, 1–17 Probiotic yogurt uses microorganisms such as ''Lactobacillus reuteri RC-14 and Lactobacillus rhamnosus ''GR-1 in the fermentation process. These probiotic bacteria play a role in promoting healthy flora growth by persisting in the intestinal tract and synthesizing organic acids that lower pH, thereby creating an intestinal environment less desirable for harmful bacteria, such as ''Salmonella spp. ''or various strains of ''E. coli. In particular, administration of probiotic bacteria has been shown to alleviate symptoms of lactose intolerance due to the auto-digesting properties of probiotic yogurt. In 2001, a study conducted by de Vrese et al. has shown that the autodigesting ability of probiotic yogurt can be partially attributed to the presence of β-galactosidase, a hydrolase enzyme that hydrolyzes the β-galactosidic bond present in lactose.Vrese, M. de, Stegelmann, A., Richter, B., Fenselau, S., Laue, C., and Schrezenmeir, J. (2001) Probiotics—compensation for lactase insufficiency. Am J Clin Nutr 73, 421s–429s Consumption of probiotic yogurt in lactose maldigesters has exhibited reduced diarrhea and flatulence. This auto-digesting feature of probiotic yogurt allows the consumption of yogurt in lactase-deficient individuals. References test